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Early Cretaceous non-marine bivalves from the Cameros and Basque-Cantabrian basins of Spain.

Bivalvos no marinos del Cretacico Inferior de las cuencas de Cameros y Vasco-Cantabrica (Espana)

1. Introduction

The so called "Wealden" facies are present in Spain in the Basque-Cantabrian Basin (Fig. 1). They developed at the northern paleo-margin of the Iberian Plate during the late Jurassic / early Cretaceous rifting stage. The remaining Spanish "Wealden" basins are located in the Iberian System: Cameros, Columbretes, Sur-iberian and Maestrazgo basins dating from the Upper Jurassic to Lower Cretaceous.

Molluscs (bivalves and gastropods) are the most abundant invertebrates in the Lower Cretaceous "Wealden" facies in Spain. Bivalves are dominant in terms of diversity, however the number of individuals is relatively low. Gonzalez Linares (1878) mentioned the existence of bivalves ascribed to Unio waldensis (sic.) Mantell (1844) and gastropods assigned to Paludina from the province of Santander. Mengaud (1920) described with little detail the bivalves and gastropods from the "Wealden" facies of the Cantabrian region. Among the bivalves, he identified U. valdensis Mantell 1844 from the locality "Casar de Periedo" and two morphotypes ascribed to "Unio" coming from "San Vicente de la Barquera". Mengaud (1920) described and illustrated one from "San Vicente de la Barquera" (page 81, fig.12) and stated it resembled Unio idubedae Palacios and Sanchez, 1885. However, neither the drawing or description corresponds to U. idubedae.

Palacios and Sanchez (1885) described Unio idubedae in a work carried out by the Spanish Geological Map Commission. The samples came from the Wealden Formation of the Soria and Logrono provinces. The authors did not designate a holotype or paratypes for the new species. However, Delvene (2005) figured the syntypes (lectotype and paralectotypes) according to the article 73.2 of the International Code of Zoological Nomenclature (I.C.Z.N.). These type specimens are housed in the Museo Geominero (Instituto Geologico y Minero de Espana, IGME). The specimens and additional field samples recently collected were ascribed to Margaritifera idubedae in previous taxonomic reviews (Bermudez-Rochas et al., 2006; Delvene et al., 2006).

[FIGURE 1 OMITTED]

Yen (1966) reviewed the molluscs of the Wealden facies, with emphasis on the gastropods of the Santander area. Amongst the bivalves, Anodonta cf. becklesi Newton (1910) and Unio cf. valdenis Mantell, 1844 were identified but no illustration were provided. Mongin (1966) studied the bivalves from different Wealden facies basins in Spain. The author described and illustrated Margaritifera (Pseudunio) idubedae (Palacios & Sanchez, 1885) and described, but did not illustrate Margaritifera (Pseudunio) valdensis Mantell (1844). However, the figured specimens of M. idubedae were unusual forms, very different from the typical shape and may belong to another species. In fact, Mongin's specimens were similar to another specimen that Palacios and Sanchez (1885) ascribed to a poorly preserved U. idubedae, or to a variety of the same species (Palacios and Sanchez, 1885: 29, Plate 6, Fig. 5).

The main objective of this work is to describe the characteristic taxonomic features of the large unionoids recorded from "Wealden" facies in the north-northwest of Spain and their paleoautecological life history traits.

2. Geological setting

The studied material originates from two different geographical areas in the north of the Iberian Peninsula (Spain): The Basque-Cantabrian Basin and Cameros Basin.

Vega de Pas 1 outcrop is located at the westernmost domain of the Basque-Cantabrian Basin, the so called Cantabrian Basin (Pujalte, 1982) or Santander Basin (Pujalte, 1989a, b). The site is situated in the Pas river bed near the town of Vega de Pas in the Province of Cantabria. The Viviparus Bed Member (Hauterivian-Barremian), is thought to represent a lacustrine environment. The sedimentological and depositional environments of the Vega de Pas Formation have been discussed in detail by Pujalte (1974; 1976; 1981; 1982).

The other area studied is in the Cameros Basin, in the northwest of the Iberian Chain. Samples came from two different lithological groups from the province of La Rioja. The Valdehierro and Valdemadera outcrops belong to the Urbion Group, dating to the upper Hauterivian-lower Barremian. The paleoecosystem is interpreted as a meandering fluvial floodplain with small temporary shallow carbonate lakes (Barrenechea, 1993). On the other hand Cornago G outcrop is part of the Enciso Group, whose paleoenvironment is interpreted as lacustrine and is considered to be Aptian (Mas et al., 2005).

3. Systematic Paleontology

All studied specimens are provisionally housed at the Museo Geominero. Specimens belonging to the collections of this museum are coded with the letters MGM, a number, and the letter C (for Cretaceous). Specimens coded with the letters that stand for the abbreviation of the outcrop's name (VH= Valdehierro, VM=Valdemadera and CG= Cornago) belong to and will be housed in one of the paleontological centres of La Rioja province when the study is finished.

A biometrical analysis has been carried out to separate the three identified species. All the figured specimens were coated with sublimated Ammonium Chloride. The asterisk indicates the type species.

Order Unionoida Stoliczka, 1871

Family Margaritiferidae Haas, 1940

Genus Margaritifera Schumacher, 1816

Margaritifera idubedae (Palacios and Sanchez, 1885) Figs. 2 and 3; Table 1

* 1885 Unio idubedae sp. nov.--Palacios and Sanchez: 28, plate, 6a, figures 4, 4a, 4b.

non 1966 Margaritifera (Pseudunio) idubedae (Palacios and Sanchez) - Mongin: 42, plate. IV figures 1, 2, 3.

non 1966 Margaritifera (Pseudunio) idubedae (Palacios and Sanchez) var. fisciformis -Mongin: 43, plate 1, figure 8.

2005 "Unio" idubedae Palacios and Sanchez--Delvene: 168, plate 1, figures 1a-b, 2a-c.

2006 Margaritifera (Pseudunio) idubedae (Palacios and Sanchez)--Bermudez Rochas, Delvene and Hernan: figures 3a-b.

Material

The type series comprises the lectotype (MGM924C) and paralectotypes (MGM941C, MGM946C, MGM1780C, MGM7414C), which are housed at the Museo Geominero (Instituto Geologico y Minero de Espana, Madrid). This material was collected by Palacios and Sanchez (1885).

Recently collected specimens: 202 from Valdehierro, coded VH1-202 (53 articulated specimens, 70 right valves, 51 left valves and 28 shell fragments) and 50 specimens, mostly fragmented, from Valdemadera (coded VM1-50).

Description

Shell solid, equivalve and inequilateral (Figs. 2 and 3), very tumid. Dorsal and ventral margins convex. Anterior part truncated, convex, rounded, and very thick. Posterior part much thinner and usually broken. Shell very thick (7.2 mm maximum). Umbones rounded, prominent and prosogyrous, the apex very acute (Fig. 2: 3). Sculpture of the umbo typically, consisting of concentric double waves, rounded in front and beaked behind, parallel to the shell's growth lines (Fig. 2: 2b, 3; Fig. 3: 2, 4a, 5). The "heart-shape" of this sculpture near the apex vanishes in the adult shell and becomes a single curve. Lunule well developed in front of the umbones (Fig. 2: 3). The hinge of Margaritifera idubedae resembles both the one of M. auricularia and that of P. litttoralis. Left valve with two massive cardinal teeth, the posterior teeth very tall and pyramidal, crenulated and well separated from the anterior tooth by a gap that is also crenulated. Anterior cardinal tooth conical, with a smooth anterior face joining the muscular scars and a posterior face with crests and holes. Anterior adductor muscle impression small, extremely rough, kidney-shaped and close to the shell margin. The anterior pedal retractor scar is excavated in the base of the anterior cardinal, just close to the adductor scar. Below the adductor impression the marks of the two pedal protractor muscles form an ear shaped mark. Right valve with only one cardinal tooth, which is very thick. Muscle impressions similar to the ones on the left valve. Lateral teeth and posterior adductor muscle scars unknown in both valves. In some of the better preserved specimens small mantle muscle scars are visible in the inner side of the valve (Fig. 3:1, 3a, 3b).

Discussion

The assignation of this species to the genus Margaritifera is based on external features, hinge characters and muscle impressions. This species' shell has an outline shape similar to the small recent Margaritifera auricularia (Spengler, 1793) and Potomida littoralis (Cuvier, 1798), although the complete shape is known from only a few specimens. The umbonal sculpture of M. idubedae seems to be similar that of P. littoralis (Bourguignat, 1864: Pl. XVIII, fig. 6), but the shell of other species like Unio or Anodonta could also present a similar ornamentation to this species (Kennard, Salisbury and Woodward, 1925). In fact, juvenile specimens of M. valdensis (Mantell, 1844) housed in Natural History Museum of London show an identical heart-shape sculpture on the umbones (Fig. 3: 6) that is also present in other Lower Cretaceous "Unio" species. The mantle muscle scars on the inner side of the valve, the arborescent rugosity and the absence of knobs on the adductor impression scars, the later being a character in P. littoralis, makes Margaritifera the most probable genus to include the species in. This genus was cited from the Wealden by Mongin (1961) including the M. valdensis Mantell (1844). M. idubedae was placed in the genus Margaritifera by Mongin (1966). However, this was based upon the study of a specimen very different from the type of Unio idubedae (Palacios and Sanchez, 1885), which does not belong to the same species. The different outline shape of the shell and the absence of the hinge in the specimen described by Mongin (1966) demonstrate that she was dealing with a different species.

[FIGURE 2 OMITTED]

Indeed, other specimens included by Mongin (1966) in the M. (Pseudunio) idubedae var. ficiformis did not correspond to M. idubedae. The main differences between idubedae and valdensis are the following: M. idubedae is shorter than M. valdensis which has a more elongated shape, the umbones of M. idubedae are more pointed than in M. valdensis where they are more rounded. Furthermore, M. idubedae has a well developed lunule and the cardinal tooth in the right valve, which is much thicker than in M. valdensis.

Occurrence. Hauterivian-Barremian (Urbion Group, Unit B) at Valdehierro and Valdemadera sites (Navajun, province of La Rioja) from Cameros Basin.

Paleoautecology. Recent Margaritifera live burrowed partially in pebble-cobble substrates in rapid flow or highly oxygenated waters. M. idubedae probably lived in a similar habitat, as it is recorded from sediments interpreted as a meandering fluvial floodplain. The recent M. auricularia live in meanders of large rivers as a filter-feeder organism.

Margaritifera valdensis (Mantell, 1844) Fig. 3: 6, Fig. 4, Fig. 5: 1-3; Table 2

* 1844 Unio valdensis sp. nov.--Mantell: 403, figures 1, 2, 3.

1846 Unio valdensis Mantell - J. de C. Sowerby: plate 646, figures 1, 2, 3.

1961 "Unio" valdensis Mantell - Mongin: 340, plate 15; plate 16, figures 1a, 1b.

[FIGURE 3 OMITTED]

? 1961 "Unio" valdensis Mantell - Mongin: plate 16, figure 2.

1966 Margaritifera (Pseudunio) valdensis Mantell Mongin: 44.

1998 Margaritifera (Pseudunio) valdensis (Mantell) Radley and Barker: figures 3a, 7.

Material

A total of 46 specimens from Cornago site (coded CGG4-1-46) coming from recent fieldworks: 33 articulated specimens, 10 left valves and 3 right valves.

Description

Shell oval, large, equivalve and inequilateral. Dorsal and ventral margins convex. Anterior part short and convex, posterior part slightly elongated but not resembling the Unio shape. Umbones rounded and obtuse, not prominent, without sculpture. Left valve with two crenulated cardinal teeth very close together, and two thick lateral teeth (only the anterior part preserved). Hinge of the right valve unknown. Muscular impression in both valves with arborescent rugosity.

Discussion

Although the anterior cardinal tooth of the studied specimen does not fit exactly with that of the types (Mantell, 1844; Mongin, 1961), the external aspect of the shell and hinge of the left valve allow us to identify our specimens as M. valdensis. This species has high morphological variability, a common feature in many unionoids. For instance the thickness of the shell and ligament depend directly on the sedimentary environments, according to Tevesz and Carter (1980) and own observations. A review of the type series indicates that the lectotype (one articulated specimen, BM3281) and syntype (the two separated valves of the same specimen, BM3282) correspond to different sediments than those of the topotype, L9744 (Mongin, 1961: plate 15). Therefore this specimen (L9744) is not considered as a topotype in the museum where it is housed (NHM), being a good example of the mentioned mophological variability of the species. The Spanish fossils belong to this second morphotype. Although Mongin (1961: 341) did not mention any umbonal ornamentation on English specimens, we did observe it on some specimens. Even if rarely visible, probably due to the acid waters that dissolve the umbo (Radley and Barker, 1998, and Munt personal communication), the ornamentation was present only on juveniles (Fig. 3: 6). Although the Spanish specimens do not preserve umbonal sculpture, this heart-like ornamentation could be a common character of both margaritiferids, M. idubedae and M. valdensis.

Occurrence. Aptian (Enciso Group) at Cornago (province of La Rioja) from Cameros Basin.

Paleoautecology.

M. valdensis is interpreted in the same way as M. idubedae, but it is recorded from some sediments that are interpreted as lacustrine deposits, where they were partially burrowed while filtering water.

[FIGURE 4 OMITTED]

Family Unionidae Fleming, 1828

Genus Protoanodonta gen. nov. Delvene and Araujo Fig. 5: 4, Figs. 6-8; Table 3

Derivatio nominis

Proto (Greek, [TEXT NOT REPRODUCIBLE IN ASCII]) = first, early Anodonta = without teeth

Type species

Protoanodonta conchae sp. nov.

Diagnosis

Shell large and flat, rounded to oval, equivalve and inequilateral, similar to Anodonta, but with a solid, flat shaped and crenulated cardinal teeth on the right valve.

Protoanodonta conchae sp. nov. Delvene and Araujo

Derivatio nominis

After Conchita Ibarrola, the mother of G. Delvene, who loved and supported the vocation of her daughter as palaeontologist.

Material

Holotype: Partial right valve (MGM10727C).

Paratypes: Right valves (MGM10728C, MGM10731C), articulated specimens (MGM10729C, MGM10730C, MGM10795C).

A total of 19 specimens (MGM10716C-MGM10733C; MGM10795C), including type material, coming from recent fieldworks and the donation of Jose Luis Sanudos fossil collection to the Museo Geominero (Instituto Geologico y Minero de Espana, Madrid): 8 right valves, 4 articulated specimens, 2 left and 5 undetermined valves.

Diagnosis

The same as the new genus Protoanodonta. Synonyms

2007 ?Margaritifera valdensis (Mantell)--Bermudez-Rochas et al.: Figure 2.

Description

Shell large and flat, equivalve and inequilateral. Shape convex to oval, dorsal and ventral margins arched. Anterior part short and convex, posterior margin convex or beaked. Shell very thin, becoming thicker in the mid-dorsal area. Umbos rounded and prominent, globose and without sculpture. Right valve with a solid, flat shaped and crenulated cardinal teeth; the remains of a poorly preserved posterior cardinal may be observed under the umbo, also the initial portion of a lateral tooth. Hinge and muscle scars of the right valve unknown. Anterior adductor muscular impression hardly observable, but probably smooth.

Discussion

Protoanodonta conchae resembles Anodonta becklesi Newton (1910: 116, plate 1, figures 1-4), but some important differences justify the necessity of defining the new genus and species. The description of Anodonta becklesi Newton, 1910 is based only on external features, and the hinge is unknown. A review of the type series and other complementary specimens of the same sediments housed in the Natural History Museum, London, show that the ascription to the genus Anodonta without checking the hinge is at best doubtful. The examination of the best preserved specimens shows that the hinge was not very strong and that the teeth were probably absent. Apart from the hinge, another main difference between A. becklesi and P. conchae is that the former has a very marked umbonal posterior carina resulting in a slight wing and in a square outline of the shell. Moreover, the shell of A. becklesi is thicker than P. conchae, although these differences can depend on environments features.

[FIGURE 5 OMITTED]

Anodonta-like specimens could have a thin shell, from heavily hinged to edentulous. The thin shell and the habitat where P. conchae was found allow us to suggest its taxonomical proximity to the genus Anodonta. Nevertheless, the presence of at least a very clear cardinal tooth and lateral teeth and the absence of umbonal sculpture make the adscription of theses shells to this genus difficult.

Type locality

Vega de Pas site (province of Santander) from BasqueCantabrian Basin.

Type horizon

Viviparus Bed Member (Vega de Pas Formation, Pas Group) Hauterivian-Barremian.

Paleoautecology

Recent Anodonta-like specimens live burrowed in a wide range of substrates, from mud to sand or gravel, sometimes in lakes and slow flowing waters. P. conchae has been recorded from sediments interpreted as lacustrine environments together with a large number of specimens of Viviparus. As other unionoids, they were filterfeeder at least in the adult stage.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

4. Conclusions

Two species of unionoids from the Lower Cretaceous at Cameros Basin have been identified, both belonging to the Family Margaritiferidae (Order Unionoida). Margaritifera idubeade (Palacios and Sanchez, 1885) is very common in the Urbion Group, which is interpreted as a fluvial system. In contrast with the English Weald where it is abundant, Margaritifera valdensis (Mantell, 1844) is an uncommon species in Spain. Here it is recorded from Enciso Group that is considered as a lacustrine environment.

Protoanodonta conchae Delvene and Araujo has been defined as a new genus and a new species of the Family Unionidae (Order Unionoida), from the Lower Cretaceous. It comes from the the Viviparus Bed Member (Vega de Pas Formation), which is interpreted as being a lacustrine environment, located at the Basque-Cantabrian Basin.

Acknowledgements

Thanks to J. Todd and M. Munt for their comments and friendly disposition for consulting the collections of Natural History Museum of London, and A. Bogan for his comments about the new taxon. The photographs were prepared by the Servicio de Fotografia Paleontologica of the Universidad de Zaragoza (Spain). The SEM photographs were made by Laura Tormo from the Museo Nacional de Ciencias Naturales (Madrid). Jose Luis Sanudo discovered the Vega de Pas site. We thank him for helping with the fieldwork and for donating his personal collection of fossils to the Museo Geominero (IGME). Joaquim Reis reviewed the English. This work is a contribution to project CGL2006-10380/BTE. We thank to the Direccion de Educacion y Cultura del Gobierno de La Rioja for permission to collect and study the material. Comments from the two reviewers (Martin Munt and J. Ramon Mas Mayoral) improved the manuscript.

Received: 04/04/08 / Accepted: 16/06/08

References

Barrenechea J.F. (1993): Evolucion de la mineralogia de arcillas en el transito diagenesis-metamorfismo de bajo grado en el Grupo Urbion (Cretacico inferior) de la cuenca de Los Cameros (Soria - La Rioja). Tesis Doctoral. Universidad Complutense de Madrid. Facultad de Ciencias Geologicas. Madrid, 299 p.

Bermudez-Rochas, D.D., Delvene, G., Hernan, J. (2006): Estudio preliminar del contenido paleontologico del Grupo Urbion (Cretacico Inferior, Cuenca de Cameros, Espana): restos ictiologicos y malacologicos. Boletin Geologico y Minero, 117 (monografico especial): 531-536.

Bermudez-Rochas, D.D., Delvene, G., Moratalla, J., Hernan, J., De la Fuente, M. (2007): Primeros datos paleontologicos del yacimiento del Cretacico Inferior Vega de Pas 1 (Cuenca Vasco-Cantabrica, Cantabria, Espana). II Semana de Jovenes Investigadores del IGME, IGME, Madrid: 23-28.

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Delvene, G. (2005): El material tipo de las especies de "Unio" (Bivalvia) del Cretacico Inferior del Museo Geominero (IGME, Madrid). Boletin Geologico y Minero, 116 (2): 167172.

Delvene, G., Araujo, R., Bermudez-Rochas, D.D. (2006): Cretaceous Spanish margaritiferids. Organisms Diversity and Evolution 6 (Electronic Supplement), 16 (1): 24. http://www.senkenberg.de/odes/06-16.htm.

Gonzalez-Linares, A. (1878): La existencia del terreno Wealdico en la Cuenca del Besaya (provincia de Santander). Annales de la Sociedad Espanola de Historia Natural, 7: 487-489.

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Mantell, G. A. (1844): On the Unionidae of the river of the country of the Iguanodon. The American journal of Science and Arts, 47 (1): 402-406.

Mas, R., Benito, M.I., Arribas, J., Serrano, A., Guimera, J., Alonso, A., Alonso-Azcarate, J. (2005): La Cuenca de Cameros: desde la extension finijurasica-eocretacica a la inversion tericaria-implicaciones en la exploracion de hidrocarburos. Zubia Monografico, 14: 9-64.

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Mongin, D. (1961): "Unio" valdensis Mantell, from the Wealden beds of England: its taxonomic position and geographical distribution. Proceedings of the Malacological Society of London, 34: 340-345.

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G. Delvene (1), R. Araujo (2)

(1) Instituto Geologico y Minero de Espana. c/. Rios Rosas, 23, 28003 Madrid. Spain g.delvene@igme.es

(2) Museo Nacional de Ciencias Naturales (CSIC). c/. Jose Gutierrez Abascal, 2. 28006 Madrid. Spain rafael@mncn.csic.es
Table 1. Measures in mm of Margaritifera idubedae (Palacios and
Sanchez, 1885)

Tabla 1. Dimensiones en mm de Margaritifera idubedae (Palacios y
Sanchez, 1885)

Specimen           Height   Shell thick

VH-2                47.4        5.5
VH-4                9.5         2.2
VH-6 (juvenile)      --         1.6
VH-8 (juvenile)      --         1.4
VH-10               59.2         7
VH-11                --         4.9
VH-13                --         7.2
VH-18               51.5         7

Table 2. Measures in mm of Margaritifera valdensis (Mantell, 1844)

Tabla 2. Dimensiones en mm de Margaritifera valdensis (Mantell,
1844)

Specimen    Length   Height   Convexity   Shell
                                          thick

CG-G4-1      73.2      --        --        2.7
CG-G4-2      42.9     31.3       10         --
CG-G4-3      70.8      --        --        5.7
CG-G4-4      85.5      53        --         --
CG-G4-5       83       58        --         --

Table 3. Measures in mm of Protoanodonta conchae Delvene and
Araujo.

Tabla 3. Dimensiones en mm de Protoanodonta conchae Delvene y
Araujo.

Specimen       Length   Height   Convexity   Shell
                                             thick

MGM10727C        104      64        --        4.4
  (holotype)
MGM10728C       98.9     62.7       --        6.2
MGM10795C       98.5      65       17.7       --
MGM10729C       66.5     57.2      15.4       --
MGM10730C      123.9     89.4      25.8       --
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Author:Delvene, G.; Araujo, R.
Publication:Journal of Iberian Geology
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Date:Jan 1, 2009
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